1. Field of the Invention
The present invention relates to anisotropic beam pumping of a Kerr lens modelocked laser. In particular, the present invention relates to using direct diode laser pumping of an ultrafast Kerr lens modelocked laser oscillator.
2. Description of Related Art
Ultrafast (i.e. pulsewidth of about 100 fs or less) lasers are used for many purposes, including seed sources for amplifiers and light sources for scientific equipment. Laser oscillators can be thought of having two main sub-modules: the pump source and the laser oscillator cavity. The pump source serves as the optical power source for the oscillator; it converts electrical power into optical power in a form that can be used to generate the desired laser output. The purpose of the laser oscillator cavity is to convert the power from the pump source into the desired optical characteristics for the given application.
Historically, Ti:sapphire has been the laser crystal of choice for ultrafast laser oscillators because of its durability, wide bandwidth, pumping ease, and commercial availability. In order for the pump source light to be converted to oscillator light, it must be absorbed by the Ti:sapphire; FIG. 1 (Prior Art) shows this absorption curve 102, as well as the fluorescence curve 104. As can be seen, the peak absorption is around 500 nm but extends from about 450 nm to 600 nm.
Initially, the Argon Ion laser was used to pump Ti:sapphire because of its emission near the absorption peak and excellent beam quality; however, the bulky, expensive, high-maintenance nature of this gas laser quickly gave way to solid state, frequency-doubled lasers such as Nd:YAG and Nd:YLF. Over the years, these solid state lasers have increased in reliability and power while decreasing in cost. However, these pump lasers all share the same complex architecture: laser diodes convert electrical power to optical power; a laser cavity produces infra-red light; and frequency-doubling crystal converts the infra-red light to a wavelength which can be absorbed by Ti:Sapphire. Inherent in this pump laser design was the assumption that the pump laser must have excellent beam quality (quantified by the M2 parameter being <˜1.5 (also called ‘diffraction limited’)) in order to achieve Kerr Lens Modelocking (KLM), the process responsible for creating the ultrashort pulses. (See, for example, Roth et al., “Directly diode-laser-pumped Ti:sapphire laser,” Opt Lett 34 3334-3336 (2009)).
Specific alternatives to the present invention include pumping ultrafast oscillators with anything that produces high beam quality in both axes such as solid-state, frequency-doubled lasers (for example those produced by Lighthouse Photonics), optically pumped semiconductor lasers (OPSELs), and gas lasers, such as those produced by Coherent Inc.
Indirect diode pumping of ultrafast oscillators using a frequency-doubled distributed Bragg reflector (DBR) tapered laser diode requires complex and expensive components to create a high beam quality output from a low beam quality laser diode source at a different fundamental wavelength.
Direct-diode pumping of ultrafast oscillators has been demonstrated before, however, these implementations either required a saturable media, such as a saturable Bragg reflector (SBR), or were accomplished with a fiber-coupled diode laser with high beam quality in both dimensions. The implementations that require a saturable Bragg reflector (SBR) for modelocking use the saturable reflection of the SBR rather than the nearly instantaneous Kerr Lens effect to implement modelocking. The advantages of Kerr Lens Modelocking over Saturable Absorbers and Saturable Reflectors are a reduction in component complexity, shorter fundamental pulse durations, and an increase in reliability. Saturable media are notorious for burning. The implementations of direct-diode pumping an ultrafast oscillator with a high beam quality in both dimensions demonstrate Kerr Lens Modelocking (P. Wasylczyk, P. Wnuk, and C. Radzewicz, “Passively modelocked, diode-pumped Yb:KYW femtosecond oscillator with 1 GHz repetition rate,” Opt. Express 17, 5630-5635 (2009).), but high beam quality fiber-coupled diodes of sufficient output power are not available at the correct wavelengths for pumping many gain media in which Kerr Lens Modelocking is advantageous, specifically Ti:sapphire. Therefore, this invention enables Kerr Lens Modelocking of ultrafast oscillators that use gain media where powerful, high beam quality diodes are not available for use as a pump laser, but where powerful diodes are available for use as a pump laser that have good beam quality in one axis but not the other.
Other alternatives include Kerr Lens Mode locked Ti:sapphire lasers and Semiconductor Saturable Absorber Mirror (SESAM) mode locked Ti:Sapphire lasers.
A need remains in the art for a laser pump scheme using anisotropic pumping of a Kerr lens modelocked laser.